1. Field of the Invention
The present invention relates to a hydraulic control system of an automatic transmission of a motor vehicle.
2. Description of the Prior Art
A known example of hydraulic control system of an automatic transmission is disclosed in Japanese laid-open Patent Publication No. 62-18336. In this hydraulic control system, a creep preventive device controls a solenoid for neutral control when the position of the select lever is changed from a neutral range to one of forward-drive ranges, so that the line pressure is supplied as it is to a start clutch (forward-drive friction engaging element) until a piston stroke is completed. After the piston stroke is completed, the clutch engaging pressure supplied to the start clutch is reduced or the supply of the pressure is temporarily stopped, so that the pressure of the start clutch is controlled to a low level at which the clutch does not have an engaging capacity.
With this arrangement, the vehicle is prevented from creeping while it is in one of drive ranges, and the supply of the line pressure can always be finished at a point of time when the start clutch is placed in the same engaging condition, thereby preventing an engaging shock and racing of the engine.
In the known hydraulic control device of the automatic transmission, however, the completion of the piston stroke is determined when a difference between the engine rotating speed and the turbine rotating speed detected at a certain moment becomes larger than a predetermined value after the range is changed. If the engine speed (Ne) changes due to an increase in the idling speed (idle up) during the piston stroke, therefore, it is possible that the piston stroke of the forward-drive friction engaging element (FWD/C) is judged by mistake as being finished at a point of time when the difference between the engine speed and the turbine speed (Nt) is increased, as shown in FIG. 4, even though the real piston stroke is not finished yet. In this case, the supply of the pressure to the start clutch is stopped or reduced. If the driver tries to start the vehicle at the time when the judgement is made by such mistake, an engaging shock or racing of the engine occurs due to a delay in engagement of the forward-drive friction engaging element, and the vehicle may not be smoothly or readily started.